Solid-state laser systems are characterized in that they have a solid-state laser gain medium that converts energy from an optical pump source to a coherent output laser beam. The pump source can be one of many available energy-producing systems such as flash lamps or semiconductor laser diodes. The energy produced by the pump source is incident upon the laser medium and absorbed by the laser medium.
The absorbed energy in the laser medium causes the atoms in the laser medium to be excited and placed in a higher energy state. Once at this higher state, the laser medium releases its own energy, which is placed into an oscillating state by the use of a laser oscillator. The laser oscillator includes at least two reflective surfaces located on either side of the laser medium. The laser oscillator may be designed to continuously release a laser beam from the system. Alternatively, the oscillator can be designed such that when the energy oscillating through the laser medium reaches a predetermined level, it is released from the system as a high-power, short-duration laser beam.
In many systems, the laser medium is Neodymium-doped, Yttrium-Aluminum Garnet (Nd:YAG). A laser medium made from Nd:YAG absorbs optical energy most readily when the energy is at a wavelength of approximately 808 nm. Thus, the source to pump the Nd:YAG laser medium should be emitting light energy at approximately 808 nm. Gallium arsenide semiconductor laser diodes can be manufactured with dopants (e.g., aluminum) that will cause the emitted light to be in a variety of wavelengths, including 808 nm. Thus, the semiconductor laser diodes, which are lasers by themselves, act as the pump source for the laser medium.
Many laser systems emit energy in a pulsed mode. To accomplish this function, a laser system may include a Q-switch that is made of a material having alterable optical properties. The Q-switch is controlled between an “opened” state and a “closed” state by a radio frequency (RF) signal, which typically operates in the range of 37 MHz to 68 MHz. When the RF signal is applied, a loss is induced in a properly aligned optical beam which passes through the Q-switch by diffracting the light off an acoustic wave inside the Q-switch material, resulting in the “closed” state. Any energy that oscillates within a laser oscillator encounters the loss produced by the Q-switch, which, if it is larger than the gain through the laser medium, prevents it from building to appreciable levels. Hence, there is no laser output power from the system when the RF signal is applied to the Q-switch. When the RF input signal is removed from the Q-switch (i.e., the “opened” state), the gain in the system overcomes the residual loss of the laser oscillator and results in a pulse of energy being emitted from the laser system.
As an example, the Q-switch can cause the laser system to produce consistent pulses of energy at a range of repetition frequencies from 1 Hz to 50 kHz. This is accomplished by quickly switching between the “opened” state and the “closed” state at this frequency. However, at the very beginning of emitting a series of pulses at high repetition frequencies, the first few pulses may be extraordinarily intense, which may be deleterious to the application. To prevent this, the RF input signal during its “opened” state may initially decrease from its maximum power level (i.e., that of the “closed” state) to zero over the first few pulses. The effect this has on laser emission reduces the initial surge in power by forcing the laser oscillator to operate temporarily in a condition of high loss, even though this loss is less than the gain of the laser medium. This Q-switch driving technique is often referred to as “first pulse suppression”.
The emitted energy produced from a solid-state laser system is generally coherent and exits the system in a predefined area. Thus, the optical power produced can be readily focused by the use of other optical components such as lenses. The resultant emitted energy can be used for a variety of industrial, medical, and scientific purposes such as cutting material, melting materials, ablating materials or vaporizing materials.
In typical systems, the traditional way to attenuate the output power from the laser oscillator is to turn down the drive current to the energy source (e.g., the laser diodes) that pump the laser medium. While the reduction in the pumping power does decrease the output power from the laser oscillator, the reduction also has undesirable effects on the output beam. In particular, the focal power of the laser medium changes, leading to a change in the fundamental mode size within the oscillator cavity. Ultimately, the output beam size, beam quality, and divergence are affected. Another way to attenuate the output power from the laser oscillator is to add additional components, such as a beam attenuator, which is located outside of the laser oscillator.
The present invention is directed to a laser system and method that solves the aforementioned problems. The output power is modulated in a manner that has only a minimal effect on the output beam size, beam quality, and divergence, without requiring additional components to be added to the laser system.